1. Field of the Invention
This invention relates to radio broadcasting and, more particularly, to methods of and apparatus for the spectral superposition of an analog AM signal and a digitally modulated signal wherein the modulation consists of amplitude and/or phase modulation of multiple mutually orthogonal, continuous valued noise-like sequences into a multidimensional constellation.
2. Description of Related Art
There has been increasing interest in the possibility of broadcasting digitally encoded audio signals to provide improved audio fidelity. Several approaches have been suggested. One such approach, set forth in copending U.S. Pat. No. 5,588,022, assigned to the assignee hereof, teaches a method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel. An amplitude modulated radio frequency signal having a first frequency spectrum is broadcast. The amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal. Simultaneously, a plurality of digitally modulated carrier signals are broadcast within a bandwidth which encompasses the first frequency spectrum. Each of the digitally modulated carrier signals is modulated by a portion of a digital program signal. A first group of the digitally modulated carrier signals lies within the first frequency spectrum and is modulated in-quadrature with the first carrier signal. Second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal. Both transmitters and receivers are provided in accordance with that method. The waveform in the AM compatible digital audio broadcasting system described in U.S. Pat. No. 5,588,022, hereby incorporated herein by reference, combines analog amplitude modulation with multiple digitally modulated carriers in the same spectrum as the analog AM signal.
In the related area of FM broadcasting, there has also been an interest in using spread spectrum noise-like sequences. However, because of the larger bandwidth of the FM channel there has been no need for methods, like multidimensional modulation, to increase the amount of information which can be transmitted.
The quality of the audio signal can be enhanced is largely dependent on the amount of data which can be transmitted through the channel. The more data that can be effectively transmitted, and also demodulated, the greater the level of audio fidelity that can be achieved. Since the bandwidth of the AM channel is relatively narrow there is a need for a method to increase the amount of data which can be effectively transmitted.